Method of magnetically and/or electrostatically positioning pressure-sensitive adhesive beads and magnetically positionable pressure-sensitive

ABSTRACT

The present invention provides pressure-sensitive adhesive beads that comprise a tacky pressure-sensitive adhesive core and a non-tacky shell material that surrounds the area wherein the beads are capable of being positioned via magnetic means. The present invention also provides method(s) of preparing adhesive coated substrates using pressure-sensitive adhesive bead(s) that are magnetically responsive, electrostatically responsive or both by using magnetic forces, electrostatic forces or both.

This is a continuation of application Ser. No. 08/028,424 filed Mar. 9,1993, (now abandoned).

FIELD OF THE INVENTION

The invention relates to a method of positioning pressure-sensitiveadhesive bead(s) comprising a pressure-sensitive adhesive core and anon-tacky shell material that surrounds the core wherein thepressure-sensitive adhesive bead(s) can be positioned via magneticmeans, electrostatic means, or a combination thereof.

This invention also relates to pressure-sensitive adhesive bead(s) whichcomprise a pressure sensitive adhesive core and a non-tacky shellmaterial that surrounds the core wherein the bead(s) are capable ofbeing positioned on a substrate via magnetic means.

BACKGROUND OF THE INVENTION

Microencapsulated adhesive beads are generally understood to comprise ashell which surrounds or encapsulates a liquid or solid adhesive core.The shell is impervious to the core material and is sufficiently strongso as to prevent exposure of the core during normal handling of thebeads. However, upon the application of heat, pressure, mechanicalforce, or the like, the shell fractures, ruptures, dissolves, or isabsorbed by the core thereby exposing the core. Microencapsulation isdiscussed in Microcapsules and Microencapsulation Techniques, by M. H.Gutcho (published by Noyes Data Corporation, Park Ridge, N.J., 1976) andMicrocapsule Processing and Technology, by A. Kondo, edited by J. W. VanValkenburg, published by Marcel Dekker, Inc., New York, N.Y., 1979.Described are limited utilities for the shell materials such as coreretention, detackification, or as a portion of the adhesive system.

Two commonly employed techniques to produce microencapsulated adhesivebeads are coacervation and in situ polymerization. In coacervation, acontinuous shell is formed when a water soluble polymer is condensedfrom an aqueous solution. The shell forms about a nucleus of materialwhich becomes the core. Shells of this type based on gelatin and gumarabic are well known.

For example, U.S. Pat. No. 2,907,682 "Adhesive Tape Containing PressureRupturable Capsules," issued Oct. 6, 1959 to H. J. Eichel discloses anadhesive tape comprising a web having a coating of two types ofpressure-rupturable capsules thereon. One type of capsule contains aliquid solvent; the other contains a substantially solid adhesive thatis soluble in the solvent. When pressure is applied to the tape, thecapsules rupture and the adhesive and solvent become mixed. The capsulesinclude a hard shell formed by coacervation from gelatin and gum arabic.These beads are coated onto the substrate in dispersion form and dried.

U.S. Pat. No. 2,988,460, "Adhesive Tape," issued Jun. 13, 1961 to H. J.Eichel discloses an adhesive tape comprising a web coated withpressure-fracturable capsules. Each capsule includes a hard shell whichsurrounds an adhesive core. The capsules are formed by coacervation andare coated in dispersion. Upon the application of pressure at atemperature substantially above 100° F. (37.8° C.), the shells fractureand the adhesive cores become tacky and flow. U.S. Pat. No. 2,988,461,"Adhesive," issued Jun. 13, 1961 to H. J. Eichel is similar to theimmediately preceding patent except that the application of pressurewithout heat is required to activate the adhesive. In this case as wellthe adhesive is coated in dispersion form.

Japanese Kokai Patent No. 63-273680, "Capsule Type Adhesive and AdhesionMethod Using Capsule Type Adhesive," Published Nov. 10, 1988, disclosesan oil with an adhesive dissolved therein. The oil is sealed in agelatin capsule. Applying pressure to the capsule causes the same tobreak thereby releasing the oil/adhesive blend. There is no mention ofthe coating technique involved using the capsule adhesive.

Japanese Patent Publication No. 60-124679, "Pressure Sensitive AdhesiveSheet" published Jul. 3, 1985 discloses several adhesive microcapsules.For example, FIG. 2(d) contained in Japanese Patent Publication No.60-124679 illustrates a pressure-sensitive adhesive core covered by afine inorganic powder and then encapsulated by a polymer film which isobtained by coacervation. Pressure is applied to the microcapsule toexpose the adhesive core. The beads are subsequently coated using asimple primer coat with spray coating of the adhesive dispersion or dustcoating of a dry adhesive material.

"In Situ" polymerization is a second commonly employed technique forproducing microencapsulated adhesive beads. A shell formed of a gaseous,liquid, water or oil soluble monomer or a low molecular weight polymeris polymerized on the surface of a core material to provide a polymerfilm which covers the entire surface of the core material. Shells basedupon urea-formaldehyde are well known. A variety of materials includinghomopolymers, copolymers, graft copolymers and block copolymers may beused to form the shell. For example, British Patent Specification No.989,264, "Microcapsules and Method of Producing Them," published Apr.14, 1965, discloses microcapsules comprising discrete, distinct andcontinuous aminoplast shell walls upon water-immiscible inert solid orliquid fill particles. No coating techniques are described. In situpolymerization is also mentioned in Japanese Kokai Patent No. 2-102280,"Microencapsulated Pressure Sensitive Adhesive Agent," published Apr.13, 1990, which discloses a pressure sensitive adhesive agent in anon-pressure sensitive adhesive shell which surrounds the agent. A dustcoating technique is employed.

Adhesive beads are also discussed in other publications. For example,U.S. Pat. No. 4,091,162, "Adhesives," issued May 23, 1978 to Hendersonet al. discloses a "core-shell" polymer particle comprising a soft,tacky polymeric core surrounded by a hard, non-tacky non-blockingpolymeric shell. The polymer shells render the adhesive beadsnon-blocking (i.e., non-agglomerating) in a latex dispersion which thenmay be coated from the dispersion, from solution, or by hot melt. Thebeads are formed by polymerizing the core followed by polymerizing theshell about the core. A typical "core-shell" polymer particle isillustrated in FIG. 1 of the Henderson et.al. patent.

Japanese Kokai Patent No. 2-102280 discloses a similar technique forproducing a structure which includes an adhesive core and a non-adhesiveshell which involves polymerizing a core followed by polymerizing theshell about the core. The above-mentioned Japanese Patent PublicationNo. 60-124679 discloses three adhesive microcapsules other than thecoacervate structure illustrated in drawing FIG. 2(d). FIG. 2(a) showsan adhesive microcapsule in which a frozen and ground pressure sensitiveadhesive is mixed with a rosin-like or terpene-like resin to form apowder that reportedly flows well at room temperature. The adhesivemicrocapsule illustrated in drawing FIG. 2(b) apparently comprises theadhesive microcapsule of FIG. 2(a) further coated with an inorganicpowder such as silica, bentonite, alumina or talc so as to enhance theflowability of the microcapsules. The adhesive microcapsule of FIG. 2(c)comprises an adhesive core coated with an inorganic powder only.

Young et al., U.S. Pat. Nos. 4,833,179 and its divisional 4,952,650,"Suspension Polymerization," issued May 23, 1989 and Aug. 28, 1990,respectively, disclose the production of non-agglomerating pressuresensitive adhesive beads by suspension polymerization. The beads includean inorganic coating of silica powder which surrounds an adhesive core.Application of the beads by hot melt coating is described.

The above-mentioned references that describe an adhesive core surroundedby a shell fail to disclose a shell which has the ability to be used inany way in the positioning of the pressure sensitive adhesive core.

Japanese Patent Publication No. 62-3192, "Powder Adhesive forElectrostatic Gravure Printing," published Jan. 23, 1987, discloses theability of a shell material to be electrostatically charged for thepurpose of gravure coating of powdered hot melt adhesives. Onlynon-pressure sensitive adhesives with limited size (5μ to 40μ), chargelevels, and charging methods (corona discharge) are disclosed. These areclaimed to be useful for electrostatic gravure printing methods only.

U.S. Pat. No. 4,427,481, "Magnetized Hot Melt Adhesive And Method ofPreparing Same," issued Jan. 24, 1984 to Mulik et.al. discussesinstalling a permanently magnetized ferromagnetic substance into a hotmelt adhesive thereby creating a dispersion. It is then formed into astrip material which in turn can be positioned prior to activation ofthe hot melt adhesive. Upon application of heat the material flows andthe magnetized particles draw the adhesive into the joint to be sealed.The patent does not teach a detackified PSA bead containing amagnetically responsive material.

Adhesive beads, in general, have been applied to substrates by a numberof means such as from dispersions, from solutions, via hot meltapplications and by dusting. Hot melt applications can be particularlydisadvantageous in that the application process may require a hightemperature which can result in the degradation of the adhesive.Furthermore, methods of pattern coating such as gravure coating can beinconvenient due to the need to substitute a new roll for each patternwhich can be time consuming as well as expensive.

SUMMARY OF THE INVENTION

A need exists for an alternative method of applying adhesive to asubstrate, particularly as a 100% solid system. A need particularlyexists for a method of pattern coating 100% solid adhesives without theinherent disadvantages of hot melt adhesive systems. We have discoveredsuch a method. A need also exists for an adhesive which can easily beapplied to a substrate via a solventless system. We have discovered suchan adhesive.

The present invention relates to a method of coating PSA beads viaelectrostatic means, magnetic means, or both, prior to their activation.The pressure-sensitive adhesive bead(s) useful according to the methodof the invention comprise a pressure-sensitive adhesive core with acontinuous or discontinuous shell coating on the surface thereof, withthe nature of the shell being that it has the ability to hold anelectrostatic charge and/or the nature of the bead being that it ismagnetically responsive such that it is useful in the positioning ortransport of the PSA bead.

The method of the invention for providing a pressure-sensitive adhesivebead coated substrate comprises the steps of:

(a) providing a substrate and a pressure-sensitive adhesive bead(s)wherein each of the bead(s) comprises a pressure-sensitive adhesive coreand a tack-free shell therearound, wherein the pressure-sensitiveadhesive bead(s) is electrostatically chargeable, magneticallyresponsive, or both;

(b) positioning the bead(s) on a substrate by a means selected from thegroup consisting of electrostatic force(s), magnetic force(s), bothelectrostatic forces and magnetic forces to form a tack-free coating ofthe bead(s) on the substrate.

The method may further comprise a step (c) of activating the bead(s) onthe substrate to expose the pressure-sensitive adhesive core and providea coating of the pressure-sensitive adhesive on the substrate.

The invention also provides an adhesive bead comprising apressure-sensitive adhesive core and a tack free shell therearound,wherein the bead is capable of being applied to a substrate via magneticmeans. The adhesive beads of the invention are environmentallyadvantageous in that they are produced via a solventless process andthus emit no solvents upon coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood with reference to thefollowing figures.

FIG. 1 illustrates a graph depicting the static voltage versus time forthe beads of Examples 1 to 11.

FIG. 2 illustrates the parallel alignment of the magnetically responsivebeads of Example 5 on a paper/magnetic tape interface prepared accordingto the procedure of Example 13.

DETAILED DESCRIPTION OF THE INVENTION

The pressure-sensitive adhesive bead useful according to the method ofthis invention comprises a core comprising a pressure-sensitive adhesiveand a shell disposed about the core. This shell may be either continuousor discontinuous in nature as long as it detackifies thepressure-sensitive adhesive core. The shell is of a nature as to be ableto have imparted an electrostatic charge to the surface and/or the shellmay be magnetically responsive and/or the core may be magneticallyresponsive.

Pressure-Sensitive Adhesive Cores

Core will sometimes be referred to herein as "the pressure-sensitiveadhesive core", it being understood that this means that the corecomprises a pressure-sensitive adhesive material. As used herein,"pressure-sensitive adhesive material" means a material which displayspressure-sensitive tack; that is, a material which is tacky when touchedwith light pressure. However, as explained below, the shell renders thebead nontacky. The PSAs which make up the PSA cores are typically of thetype which would provide useful PSA coated materials such as sheetmaterials, (e.g., tapes, labels, and the like) metals, plastics,ceramics, etc.

The pressure-sensitive adhesive core can comprise a variety ofadhesives. The pressure-sensitive adhesive cores may be derived fromadhesives including but not limited to those selected from the groupconsisting of polyacrylates, conventional rubbers including but notlimited to those selected from the group consisting of natural rubbers,styrene-butadiene block copolymers, elastomeric rubbers such as butylrubber and poly(alpha-olefins), and blends thereof. Plasticizers and/ortackifiers are generally added to produce the desired pressure-sensitiveadhesive properties. Still other suitable adhesive cores include thoseselected from the group consisting of vinyl ether polymers and siliconepressure-sensitive adhesives, both of which may be blended with acrylicpressure-sensitive adhesives or prepared as acrylic copolymers. For allof these PSA cores the same shell coatings may be employed.

The diameter of the PSA core can vary depending upon the applicationdesired. Typically the diameter of the PSA core ranges from about 10microns to about 3200 microns, preferably about 25 to about 500 microns.Typically, beads having a smaller core diameter will provide a morecontinuous PSA coating on a substrate on which they are coated andactivated. Conversely, beads having a greater core diameter will providea more discontinuous coating on a substrate on which they are coated andactivated. However, beads having a smaller core diameter tend to have ahigher shell to core ratio thus resulting in a coating which provideslower adhesion values.

The pressure-sensitive adhesive cores may be prepared by a number oftechniques. For example, the PSA cores may be prepared by granulating abulk pressure sensitive adhesive material at low temperatures (e.g.,about -60° C. or below). The pressure-sensitive adhesive may be frozenwith liquid nitrogen so as to detackify the adhesive and then ground toprovide granular matter useful as the adhesive cores. In addition, thePSA cores may be prepared by an aqueous suspension technique, such asthe cores described below.

A polyacrylate pressure sensitive adhesive core may be prepared by anaqueous suspension polymerization process similar to that disclosed inU.S. Pat. No. 4,833,179 and U.S. Pat. No. 4,952,650, which patents areboth incorporated by reference herein. In general, the suspensionpolymerization technique described in these patents involves the stepsof:

(1) preparing a monomer premix comprising:

(a) acrylic acid ester(s) of a non-tertiary alcohol, the alcohol havingfrom 1 to 18 carbon atoms, with the average number of carbon atoms beingabout 4 to about 12;

(b) a functional monomer copolymerizable with the acrylic acid ester;

(c) a free-radical initiator; and

(d) a chain transfer agent;

(2) combining the premix with a water phase containing a dispersion aidand/or a stabilizer to form a suspension; and

(3) concurrently agitating the suspension to permit the polymerizationof the monomer premix until the pressure-sensitive adhesive cores form.

Alkyl acrylate monomers (i.e. acrylic acid ester monomers) useful inpreparing the pressure-sensitive adhesive include but are not limited tomonofunctional, unsaturated acrylate ester monomers. Included withinthis class of monomers are, for example, isooctyl acrylate, isononylacrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate,n-butyl acrylate, hexyl acrylate, and mixtures thereof. The acrylatemonomers comprise at least about 70 parts by weight based on 100 partsby weight total monomer content, preferably from about 75 parts byweight to about 90 parts by weight. Unless indicated otherwise, allparts are parts by weight.

Alkyl fumarates and alkyl maleate (based, respectively, on fumaric andmaleic acid) may also be successfully used. Examples thereof include butare not limited to those selected from the group consisting of dibutylmaleate, dioctyl fumarate, dibutyl fumarate, and dioctyl maleate.

The functional monomer copolymerizable with the acrylic acid ester, thefumaric acid ester, or the maleic acid ester is incorporated into themonomer premix so as to modify a final property (for example, peeladhesion or shear holding strength) of the resulting adhesive core. Thefunctional monomer may be a polar monomer. "Polar monomers" include bothmoderately polar and strongly polar monomers. Polarity (i.e.,hydrogen-bonding ability) is frequently described by the use of termssuch as "strongly," "moderately" and "poorly." References describingthese and other solubility terms include "Solvents," Paint TestingManual, 3rd Ed., G. G. Seward, Editor, American Society for Testing andMaterials, Philadelphia, Pa., and "A Three-Dimensional Approach toSolubility," Journal of Paint Technology, Vol. 38, No. 496, pp. 269-280.Strongly polar monomers useful herein include acrylic acid, methacrylicacid, itaconic acid, hydroxyalkyl acrylates, styrene sulfonic acid orthe sodium salt thereof, maleic acid, fumaric acid, citraconic acid,acrylamides, and substituted acrylamides. Moderately polar monomersuseful herein include N-vinyl lactams such as N-vinyl pyrrolidone,N-vinyl caprolactam, acrylonitrile, and dimethyl amino-propylmethacrylate.

Other monomeric materials which may be used as the functional monomerinclude macromers of the type disclosed in U.S. Pat. No. 3,786,116 suchas 1-polystyrylethyl methacrylate, vinyl esters such as vinyl acetateand vinyl chloride, dialkyl maleate such as dioctyl maleate and dibutylmaleate, dialkyl fumarates such as dioctyl fumarate and dibutylfumarate, and alkyl methacrylates such as methyl methacrylate.

Mixtures of any of the above noted functional monomers may also beemployed. For example, a preferred functional monomer comprises a blendof vinyl acetate, methyl methacrylate and methacrylic acid. Thefunctional monomer may comprise up to about 30 parts by weight of thepremix based on the total monomer content, preferably from about 10 toabout 25 parts by weight.

Initiators for polymerizing the monomers to provide the adhesive coresof the invention are those which are normally suitable for free-radicalpolymerization of acrylate, fumarate and maleate monomers and which areoil-soluble and have low solubility in water, which include but are notlimited to those selected from the group consisting of organic peroxidessuch as benzoyl peroxide, lauryl peroxide and various thermalinitiators. An example of a useful thermal initiator is2,2'-azobis(isobutyronitrile), commercially available from E. I. dupontde Nemours & Co. (Wilmington, Del.) under the tradename VAZO™64. Theinitiator is present in an amount ranging from about 0.05 to about 1part by weight based on 100 parts by weight total monomer content.

In the course of carrying out the suspension polymerization of theseadhesive cores, chain transfer agents, including but not limited tothose selected from the group consisting of mercaptans, alcohols, andcarbon tetrabromide, may be useful. Representative examples of usefulchain transfer agents include those selected from the group consistingof isooctyl thioglycolate, carbon tetrabromide, etc. The chain transferagent is present in an amount ranging from about 0.01 to about 0.5 partby weight based on 100 parts by weight total monomer content.

If aqueous suspension polymerization is used to prepare these adhesivecores, conventional dispersion aids, stabilizers and, optionally,anionic and nonionic surfactants may be advantageously employed. Theamount of surfactant, if included, is preferably from about 2.5 partsper million to about 1.0 part by weight based on 100 parts per weighttotal monomer content. Representative examples of useful surfactantsinclude those selected from the group consisting of sodium laurylsulfate, sodium dioctyl sulfosuccinate, and mixtures thereof.

Dispersion aids are those conventionally used in suspensionpolymerization processes. Typically they are water insoluble orminimally water soluble inorganic powders including but not limited tothose selected from the group consisting of tribasic calcium phosphate,calcium carbonate, calcium sulfate, barium sulfate, barium phosphate,hydrophilic silicas, zinc oxide, magnesium carbonate, and mixturesthereof.

Typical stabilizers are water soluble organic compounds, including butnot limited to those selected from the group consisting of polyvinylalcohol, poly-N-vinyl-2-pyrrolidone, polyacrylic acid, polyacrylamide,hydroxyalkyl cellulose, and mixtures thereof. Poly-N-vinyl-2-pyrrolidoneand polyvinyl alcohol with a viscosity based molecular weight of about15,000 to about 630,000 are preferred. The total amount of dispersionaid and stabilizer is present in an amount ranging from about 0.01 partto about 5 parts by weight based on 100 parts per weight total monomercontent.

Optionally, photocrosslinking agents may be used in preparing theadhesive cores of the invention. Representative examples of usefulcrosslinking agents include copolymerizable aromatic ketone monomers,such as acryloxybenzophenone. When present, the photocrosslinkergenerally comprises from about 0.01 part to about 5 parts by weightbased on 100 parts by weight total monomer content.

Various additives may also be included in the monomer premix. Suchadditives include, for example, bases including but not limited to thoseselected from the group consisting of ammonia, tertiary amines, sodiumhydroxide, barium hydroxide, calcium hydroxide, magnesium hydroxide,potassium hydroxide, lithium hydroxide, and mixtures thereof. Theseadditives comprise from about 0.1 part to about 5 parts by weight basedon 100 parts by weight total monomer weight.

According to one method of making the polyacrylate PSA cores themonomers, free-radical initiator, chain transfer agent, and otheradditives (if included) are blended in the prescribed ratio to form amonomer premix. The monomer premix is then combined with an aqueousphase comprising water, a dispersion aid, a stabilizer, any optionalsurfactants (all as discussed more fully herein above) and polymerized,with agitation, for about 1 to 10 hours at a temperature of about 45° C.to about 85° C. to give a suspension which contains the preferredadhesive cores. The cores may be washed and separated from the water bymeans such as gravity filtration. The filtered product generallycomprises about 15 to about 30 percent by weight water. The resultingadhesive cores typically have a diameter of about 10 microns (μ) toabout 3200 microns and are usually pearl shaped.

Pressure-Sensitive Adhesive Bead Shells

The pressure sensitive adhesive core has a non-tacky shell disposedtherearound. As indicated previously, the core may be coated with ashell comprising an electrostatically chargable material and/or the coremay be coated with and/or impregnated by a magnetically responsivematerial.

The shell material of the bead may be used to initially position the PSAbead on the ultimate substrate to be coated or on a first substratewhich serves as a transfer medium. If desired one may use a series oftransfer media. This positioning on the ultimate substrate and/or thetransfer media may be accomplished by the use of electrostaticallychargeable shell materials, such as those in the triboelectric series,and/or magnetically responsive materials which are embedded and/ordispersed about the PSA core. Normally, the presence of a static chargeon an adhesive bead would be considered a hindrance and thereforeundesirable, but we have discovered a variety of novel coatingtechniques which make use of this heretofore undersirablecharacteristic. We have also discovered novel coating techniques whichmake use of our novel magnetically responsive beads.

One form of shell coating is considered to be essentially discontinuous.By "essentially discontinuous" it is meant that the shell coatingcomprises a multiplicity of discrete particles which substantiallysurround the inherently tacky core such that the core is notsubstantially exposed. Another form of shell coating is considered to beessentially continuous. By "essentially continuous", it is meant thatthe shell while perhaps containing fissures or cracks therein does notcomprise a multiplicity of discrete particles which substantiallysurround the adhesive core but rather a substantially continuous shell.

The particles which surround the core to provide an essentiallydiscontinuous shell are substantially uniform in size and shape. Theparticles may be provided in a single layer or more than one layer aboutthe core or may be provided in groups or clusters which cooperate so asto substantially surround the core. By "substantially surround" and "notsubstantially exposed" it is recognized that gaps or spaces may existbetween individual particles (or clusters thereof) so long as thesurface of core is not exposed to a degree that renders beads not freeflowing as explained more fully herein below.

The shell materials can be applied in in-situ polymerization, latex orsolvent dispersion form, or as a granulated powder. When the ,shellmaterials are applied as a granulated powder the pressure-sensitiveadhesive cores can be coated by a variety of methods such as by dustingthe core with or rolling the core in the granulated powder.

Electrostatically Chargeable Shell Materials

Useful electrostatically chargeable shell materials include a widevariety of non-tacky materials including but not limited to thoseselected from the group consisting of non-tacky thermoplastic polymers;natural polymers including but not limited to those selected from thegroup consisting of wool, silk, celluloses such as cotton and linen,starch, gelatin, polysaccharides such as agar and carrageenan, etc.;thermosetting polymers including but not limited to those selected fromthe group consisting of urea-formaldehyde resins,phenol/resorcinol-formaldehyde resins, melamine-formaldehyde resins;epoxy resins; alkyd resins; organic compounds which can be made inpowdered form including but not limited to those selected from the groupconsisting of rosin esters, terpenes; and electrostatically chargableinorganic materials including but not limited to those selected from thegroup consisting of silica, titanium dixoide calcium carbonate,ceramics, talc, kaolin, clay, mineral powders such as quartz, asbestos,galena, gypsum, and the like.

According to one method of providing adhesive cores having anelectrostatically chargeable and/or magnetically responsive shell so asto form adhesive beads according to the invention, a polymeric materialmay be combined with the aqueous suspension of formed adhesive cores (ifthe cores are formed by an aqueous suspension polymerization). Anexample of such a polymeric material is an organic thermoplastichomopolymer or an organic thermoplastic copolymer derived from a latexof the homopolymer or the copolymer (collectively referred tohereinafter at times as "an organic polymer latex") or derived from asolvent dispersion of the homopolymer or the copolymer (collectivelyreferred to hereinafter at times as "a solvent dispersion of an organicpolymer"). Magnetically receptive or magnetized particles (magneticallyresponsive particles) may or may not be added to the solvent dispersionof organic polymer latex and then dried and ground for the purpose ofcreating seed particles for the formation of the adhesive cores or forimproved attraction of the coated particles during post treatment of theadhesive cores. Alternatively, the material from which the shell isformed may be provided as a granulated powder which may optionallyencompass magnetically receptive particles.

By "thermoplastic" is meant a material that is capable of beingrepeatedly softened by heat and hardened by cooling over a particulartemperature range. By "thermosetting" it is meant a material that iscapable of being rendered hard by the application of heat. "Latex"refers to an aqueous dispersion of the particular material which istypically produced by emulsion polymerization. By "copolymer" is meant apolymeric material comprised of two or more monomers.

One type of polymer shell of the invention can comprise monomers ormixtures thereof which are polymerized by a free-radical polymerizationprocess such as emulsion, suspension, or bulk polymerization. When thepolymer shell is derived from emulsion or suspension polymerizationprocesses using redox (reduction-oxidation) or thermally activatedinitiators, it is provided in the form of an organic polymer latex.Preferably, the polymer additive is provided as a latex having particleswith a diameter of 10μ or less. If desired, the latex may be dried andground to provide the polymer additive in powder or granulated form.This powder may be used to detackify the adhesive cores. Alternatively,the powder can then be redispersed in an appropriate organic solvent soas to provide a solvent dispersion of the organic polymer.Alternatively, the organic polymer latex may be dissolved in anappropriate organic solvent. Suitable organic solvents include lowpolarity alcohols such as isopropanol and n-butanol, aliphatichydrocarbon solvents such as hexane and heptane, aromatic hydrocarbonsolvents such as benzene, toluene and xylene, as well astetrahydrofuran, methyl ethyl ketone, and the like.

Free-radical bulk or solution polymerization requiring thermal orphotochemical initiation using organic peroxides, hydroperoxides, azo ordiazo compounds may be employed. Other polymerization processes such ascationic, anionic and coordination polymerizations can also provide thepolymer shell. References which discuss such processes include F. W.Billmeyer, Textbook of Polymer Science, 3rd Ed., Wiley, Interscience1984, pp. 85-91, incorporated by reference herein, and R. Morrison andR. Boyd, Organic Chemistry., 3rd. Ed., Allyn and Bacon, 1973, pp.1037-1039, incorporated by reference herein.

Cationic polymerization is preferably limited to unsaturated hydrocarbonpolymer coatings such that Lewis acids, protonic acids or carbenium ionsare typically used as catalysts along with low reaction temperatures.(i.e., usually below room temperature). Anionic polymerization (alsoknown as "living" polymerization) is typically initiated by stronganions derived from alkyl lithium, sodium in liquid ammonia, and thelike at room temperature or below to give essentially monodispersepolymers. Coordination polymerization involves Ziegler-Natta catalystsusually employed in fluidized bed processes to give stereospecificpolymers. The polymer produced by any of these methods is usually inbulk or semi-bulk form after removal of the solvent or carder, if anywas used. Granulation of the resulting product provides the polymeradditive in micronized powder form.

Suitable monomers for the formation of either the thermoplastichomopolymer or copolymer include but are not limited to those selectedfrom the group consisting of styrene, vinyl acetate, vinyl chloride,vinylidene chloride, alkyl methacrylates such as methyl methacrylate,ethyl methacrylate or butyl methacrylate and mixtures thereof (i.e.,vinyl group and acrylate group containing materials). When the polymeradditive is provided as a thermoplastic copolymer, the above monomersmay be blended with each other and/or further mixed with a polarcomonomer including but not limited to those selected from the groupconsisting of sodium styrene sulfonate, sodium acrylate, sodiummethacrylate, acrylic acid, methacrylic acid, maleic acid, fumaric acid,sodium maleate, sodium fumarate, citraconic acid, vinyl betaines,N-vinyl-2-pyrrolidone, 4-vinylpyridine, acrylamides, substitutedacrylamides, and mixtures thereof. Preferred polar comonomers includesodium styrene sulfonate, acrylic acid, sodium acrylate, methacrylicacid, sodium methacrylate, N-vinyl-2-pyrrolidone and acrylamide. Whenpresent, the polar comonomer comprises from about 0.5 part to about 10parts by weight based on 100 parts by weight total monomer content ofthe organic copolymer coating.

Both the adhesive cores and the shell coatings may contain one or moreadjuvants. Preferred adjuvants include those selected from the groupconsisting of tackifiers, plasticizers, pigments, dyes, extenders,fillers, antioxidants, multifunctional crosslinkers, stabilizers,magnetically responsive materials (discussed infra), and mixturesthereof. An especially preferred additive is bis-vinyl ether whichprovides high cohesive strength. When present, this additive generallycomprises from about 0.5 to about 1 part by weight based on 100 parts byweight total adhesive core monomer content.

Preferably, the thermoplastic homopolymer or copolymer has a glasstransition temperature (Tg) of at least about 25° C., more preferablyfrom about 90° to about 95° C., while the adhesive core preferably has aTg below about 20° C. If the Tg of the homopolymer or copolymer of theshell is less than about 25° C., it may be too close to the Tg of theadhesive core material and, as a result, may tend to blend therewith andpossibly materially adversely affect the free-flowing quality of theadhesive beads of the invention.

The following are several specific methods of providing the PSA coreswith a shell. The shell material whether provided as an organic polymerlatex, a solvent dispersion, or as a powder, may be combined with thepressure-sensitive adhesive cores by several different techniques so asto form adhesive beads according to the invention. In each instance, theshell material is provided in an amount ranging from about 1 part toabout 5 parts per 100 parts by weight of adhesive cores, preferably fromabout 2 parts to about 3 parts.

According to one technique an organic polymer latex derived by emulsionpolymerization may be combined with previously formed and filteredadhesive cores and then agitated so as to provide the adhesive coreswith an essentially discontinuous organic polymer shell derived from thelatex. Alternatively, in situ emulsion polymerization may be employed toprepare a discontinuous coating. According to this approach, theingredients which provide the shell are blended together and emulsionpolymerized in the presence of the previously formed adhesive cores.According to a third technique of preparing a discontinuous shellcoating, the ingredients which provide the shell may be added to theadhesive core monomer premix after the exotherm which occurs during thesuspension polymerization thereof. The shell materials may then beemulsion polymerized. As a fourth alternative of producing adiscontinuous shell coating, an organic polymer latex for forming theshell may be combined with the adhesive core monomer premix prior to thesuspension polymerization of the cores. Such an approach may be regardedas an "in-line" process.

Magnetically Responsive Materials

The beads of the invention can comprise one or more magneticallyresponsive materials to aid in positioning the beads. The term"magnetically responsive materials" as used herein refers to materialswith sufficient magnetic attraction so as to be useful in the transportof the PSA bead of the invention onto a transfer medium and/orsubstrate. Magnetically responsive materials include but are not limitedto ferromagnetic materials, salts of ferromagnetic materials, and alloysof ferromagnetic materials, which may or may not be magnetized. Forexample, a ferromagnetic material may be permanently magnetized to forma magnetically responsive material which is a permanent magnet.

Examples of useful ferromagnetic materials include but are not limitedto those selected from the group consisting of iron, cobalt, nickel,gadolinium, dysprosium, including alloys and salts of these metals withother elemental materials including but not limited to those selectedfrom the group consisting of carbon, silicon, aluminum, copper,chromium, manganese, magnesium, titanium, barium, strontium, tungsten,vanadium, niobium, platinum, and silver. Nickel is a preferredferromagnetic material due to its low cost and low reactivity. Iron isless useful due to its tendency to oxidize.

It is possible to coat the magnetically responsive materials with amaterial to enhance the affinity of the magnetically responsive materialto the PSA core. Such coating is of increased importance when asuspension polymerization is employed in forming the PSA cores since thewater phase can cause the magnetically responsive material to phaseseparate. Preferably, the magnetically responsive particle(s) is coatedwith a thermoplastic material. Useful thermoplastic materials includebut are not limited to those selected from the group consisting of adispersion of polymeric thermoplastics, such as those derived fromacrylates, polyolefins, polystyrenes, and the other shell materialdiscussed infra.

The number and size of the magnetically responsive particles containedwithin the PSA core or the shell or both can vary. One magnetic particlemay be sufficient. However, one magnetic particle would not besufficient to provide the PSA core with a non-tacky shell. The remainderof the non-tacky shell would thus need to be provided by a non-tackyelectrostatically chargeable material (discussed Supra) and/or a neutralinert organic or inorganic non-tacky material which is neitherelectrostatically chargeable nor magnetically responsive which does notinterfere with the PSA properties of the PSA core. Examples of suchneutral inorganic materials include but are not limited to thoseselected from the group consisting of inorganic salts including but notlimited to those selected from the group consisting powders of bariumsulfate, sodium chloride, calcium sulfate, calcium chloride, sodiumsulfate, sodium phosphate, calcium phosphate, mixtures thereof, and thelike.

The size and number of the magnetic particles if used without anytriboelectric materials must be sufficient to allow for the transfer ofthe adhesive bead of the invention onto a transfer medium if used,and/or onto the ultimate substrate to be coated. If a large amount ofmagnetic material is employed it can interfere with the adhesiveproperties of the PSA coating prepared from the beads. If a very largeamount of magnetic material is included the bead will function more orless as a means for adhering a magnetic material to a substrate ratherthan as a means for adhering a PSA to a substrate.

The magnetic particle size can vary. Typically, each magnetic particlehas a smaller diameter than the PSA cores which they surround and/or areembedded in. Typically each magnetic particle(s) has a diameter of about1 to about 100 microns.

Methods of Drying Beads

Once the basic bead structure comprising the pressure sensitive adhesivecore and the non-tacky electrically responsive shell and/or magneticallyresponsive core and/or magnetically responsive shell has been formed,the beads, if contained in a suspension, may be dried so as to convertthe beads to an essentially moisture-free condition. By "essentiallymoisture-free" it is meant that the beads contain no more than about 5 %moisture. Any of a variety of conventionally used drying methods suchas, for example, freeze drying, heated air flash drying, spray drying,fluidized bed drying or column drying may be employed. Spray drying is aparticularly preferred technique. The beads may be filtered prior todrying using, for example, a bulk dewatering process such as a beltconveyer.

Methods of Coating Beads

Pressure-sensitive adhesive beads useful according to the method of theinvention are those which are useful in any application in whichpressure-sensitive adhesive would have utility. Preferably, the beadsare applied to a substrate by coating them as a 100% solids system.Prior to, during, or subsequent to the application of the adhesive beadsto the final substrate the beads are activated to expose thepressure-sensitive adhesive cores. The beads can be activated by anumber of methods including but not limited to the application of heat,the application of pressure, or both. When the beads are in a dry,free-flowing condition the core and the coating may be regarded asnon-homogeneous in the sense that the core and the coating are notblended with each other at room temperature (about 20°-22° C.). However,upon the application of heat and/or pressure, the adhesive cores melt orcold flow thus becoming exposed and form a blended adhesive coating. Anexample of simultaneous heat and pressure activation is the passing of abead coated substrate between a pair of heated nip rollers or the like.

Conventional pressure sensitive adhesives of 100% solids formulationsare packaged in drums, pails or cartons. Because of the inherently tackynature of the adhesives, release liners or special unloaders must beemployed to evacuate the adhesive from its packaging. Adhesive beadsuseful according to the invention function as a conventional pressuresensitive adhesive subsequent to activation but prior to activation arefree from these packaging disabilities due to the free-flowing nature ofthe beads.

The beads may be coated by any means employing electrostatic charges(such as triboelectric charges), magnetics, or a combination thereof.Electrostatic charges can be applied to the bead via conduction, coronatreatment, photoconduction charging, and the like. Triboelectric chargesare defined as charges which are imparted to the surface of the bead byfriction. This may be accomplished by a mechanical process such astumbling, brushing, air conveyance, or the like. One of the uniquefeatures of the electrostatically chagerable beads described are theirability to accept either a positive or negative charge. This isaccomplished by the method of charging and the electron donating orelectron withdrawing ability of the shell polymer or copolymer. Thiscapability allows the broadest possible coating methods to be employed,independent of the charge required. The triboelectric series can beconsulted to determine which of two substances would become negativelycharged and which would be positively charged when the two are rubbedtogether. The triboelectric properties of certain polymers in descendingorder of electron donorship from positive (donor) to negative (acceptor)are as follows: nylon 6,6 (also wool, silk), cellulose, celluloseacetate, polymethyl methacrylate, polyacrylonitrile, polyvinyl chloride,polybisphenol carbonate, polychloroether, polyvinylidene chloride,poly-2,6-dimethyl polyphenylene oxide, polystyrene, polyethylene,polypropene, and polytetrafluorothylene. The role played by thetribolelectric series is that, it allows prediction of the polarity andlevel of charge obtainable.

The beads useful according to the invention may be applied to asubstrate via a number of electrostatic processes such aselectrostatographic processes (electrographic, electrophotographic,combinations thereof, etc.)

An electrostatic charge opposite to that applied to the beads may beapplied to the substrate. and also to a transfer medium, if used, by anumber of methods including but not limited to the following: brushing,conduction, corona treatment, and photoconduction.

A repulsive electrical field may be generated which can aid inpositioning the charged beads on the transfer medium, if used, and alsoon the substrate. The repulsive electrical field(s) can be generated bya number of methods including but not limited to the following:brushing, conduction, corona treatment, and photoconduction. Therepulsive field is of sufficient magnitude and positioned such that acharged bead placed therein will be transported to the transfer medium,if used, and/or depending upon the method of application transported tothe substrate.

When the pressure-sensitive adhesive beads are electrostaticallychargeable the pressure-sensitive adhesive bead(s) may be positioned ona substrate by attracting and contacting the bead(s) to the substrate bymeans of an electrostatic force.

The electrostatic force may be applied by a wide variety of methods.Included herein are several specific examples, of the use ofelectrostatic forces to apply beads to a substrate. However, thisinvention is not limited to these specific examples.

As one example, an electrostatic charge may be applied on the substrateopposite to an electrostatic charge generated on the pressure-sensitiveadhesive beads. The charged pressure-sensitive adhesive beads arebrought close to the charged substrate so that the beads are attractedto and contact the substrate and form a tack-free coating on thesubstrate due to the electrostatic attraction. Alternatively, anelectrical field encompassing the substrate may be generated and anelectrostatic charge applied to the pressure-sensitive adhesive beads.The electrical field which, in this example, is repulsive to the chargedpressure-sensitive beads is of sufficient strength and is positionedsuch that it is capable of positioning the charged beads placed thereinon the substrate. The charged pressure-sensitive adhesive beads arebrought into the field so that the pressure-sensitive adhesive beadscontact the substrate and form a tack-free coating on the substrate.Combinations of two or more methods may also be employed.

When each of the pressure-sensitive adhesive beads is magneticallyresponsive the beads may be attracted to and contacted with thesubstrate by means of one or more magnetic forces thereby positioningthe pressure-sensitive adhesive beads on the substrate to form atack-free coating of the pressure-sensitive adhesive beads thereon.

The magnetically responsive beads of the invention may be applied to asubstrate via a magnetic process such as a magnetographic process. Amagnetic field may be generated around the transfer medium, if used,and/or around the substrate depending on the nature of the beads and thenature of application desired. The magnetic field can be generated, forexample, by a permanent magnetic and/or by an electrically inducedmagnetic field.

Included herein are several specific examples of the use of magneticforces to apply beads to a substrate. However, the invention is notlimited to these specific example.

As one example, when the magnetically responsive pressure-sensitiveadhesive beads comprise permanent magnetic particle(s) the magneticattraction force can be provided by a magnetically responsive materialin the substrate (i.e., the substrate can comprise a magneticallyresponsive material) or a magnetically responsive material can bepositioned on a side of the substrate opposite the pressure-sensitiveadhesive beads. The pressure-sensitive adhesive beads are brought closeto the substrate thereby allowing the magnetic force to position thepressure-sensitive adhesive beads on the substrate to form a tack-freecoating thereon. As another example, when the magnetically responsivepressure-sensitive adhesive beads do not comprise permanent magneticparticles a magnetic attraction force may be provided by a magneticfield encompassing the substrate. The pressure-sensitive adhesive beadswithin the magnetic field are thereby positioned on the substrate toform a tack-free coating thereon. Combinations of methods may also beemployed.

Regardless of the method of application the beads can be activated by anumber of methods including but not limited to the application of heat,pressure, or both heat and pressure to provide a coating of PSA on thesubstrate.

When a transfer medium is used, the adhesive beads may be activatedprior to transfer to a second substrate, subsequent to transfer to asecond substrate, or simultaneously with transfer to the secondsubstrate. Thus, in one situation, the pressure-sensitive adhesive beadson the transfer medium may be activated to provide a coating ofpressure-sensitive adhesive on the transfer medium. Thepressure-sensitive adhesive is then transferred to a second substrate toprovide a coating of pressure-sensitive adhesive on the secondsubstrate. In another situation, the pressure-sensitive adhesive beadsmay be transferred to a second substrate by a means selected from thegroup consisting of magnetic means, electrostatic means, and bothmagnetic means and electrostatic means and simultaneously activated,thus exposing the cores of the pressure-sensitive adhesive beads to forma coating of pressure-sensitive adhesive on the second substrate. Instill another situation, the pressure-sensitive adhesive beads may betransferred to a second substrate by a means selected from the groupconsisting of magnetic means, electrostatic means, and both magneticmeans and electrostatic means, following which the beads aresubsequently activated thereby exposing the pressure-sensitive adhesivebeads to form a coating of pressure-sensitive adhesive on the secondsubstrate.

This may be done by a variety of methods. For example, an electricalfield repulsive to the beads may be generated such that it encompassesthe transfer medium. The electrical field is positioned and is ofsufficient strength to position the charged beads placed in theelectrical field on the transfer medium. Optionally, a charge oppositeto that on the beads can be placed on the transfer medium.Alternatively, the transfer medium can be charged and placed in theabove-mentioned electrical field. The beads are then brought close tothe transfer medium and/or the optional charge on the transfer mediumand/or within the optional fields encompassing the transfer medium inorder to transfer the beads onto the transfer medium forming a tack-freecoating thereon. The beads can subsequently be activated on the transfermedium.

As another example, a second substrate, which has been optionallycharged with an electrostatic charge opposite to that on the beads, canbe brought into contact with unactivated beads on the transfer medium,and simultaneous activation of the beads can occur (by the applicationof heat, pressure, or both heat and pressure, for example) so as toexpose the pressure-sensitive adhesive core and form a coating of apressure-sensitive adhesive on the second substrate.

As another example, a second substrate which has been optionally chargedwith an electrostatic charge opposite to that on the beads can bebrought close to the beads on the transfer medium. The beads can then beallowed or caused to contact the second substrate and form a tack-freecoating on the second substrate by various methods depending on theforce holding the beads to the transfer medium. This may involve one ormore of the following: removing the optional charge from the transfermedium; removing the repulsive electrical field keeping the beads incontact with the transfer medium; applying an electrical force repulsiveto the beads on a side of the transfer medium opposite the beads;providing the second substrate with an electrostatic charge greater thanthat that has optionally been applied to the transfer medium. The beadscan subsequently be activated after transfer to the second substrate soas to expose the pressure-sensitive adhesive core and form a coating ofa pressure-sensitive adhesive on the second substrate.

In one situation, when magnetics are involved in holding the beads tothe substrate, one can bring a second substrate, which optionally hasencompassing therearound a magnetic field, into contact with the beadson the transfer medium, simultaneously activating the beads so as toexpose the pressure-sensitive adhesive core and provide a layer of apressure-sensitive adhesive on the second substrate.

In another situation when magnetics are involved, one can bring a secondsubstrate, which optionally has encompassing therearound a magneticfield of greater magnitude than that around the transfer medium, closeto the beads on the transfer medium, so that the beads contact thesecond substrate due to the stronger magnetic forces. This results in atack-free coating of the beads on the second substrate. This can befollowed by the subsequent step of activating the beads so as to exposethe pressure-sensitive adhsive core and provide a coating of apressure-sensitive adhesive on the second substrate.

Examples of transfer media for electrostatic processes include but arenot limited to those materials that will induce an electrical charge andhold the charge for a working period (insulators). Examples thereofinclude but are not limited to those materials selected from the groupconsisting of thermoplastics, wood, paper, impregnated cloth such asepoxy siliconized cloth, rubberized cloth, etc. Useful transfer mediafor magnetic processes include but are not limited to those that allowthe passage of a magnetic field therethrough or those that may bemagnetized themselves. The transfer medium can take the form of a thinmetallic film, drum, roll, metallized film, cloth, metallized cloth,etc.

Combinations of electrostatics and magnetics can also be employedaccording to the invention. Included herein are several specificexamples. However, the invention is not limited to these specificexamples. Such a method may, for example involve providing apressure-sensitive adhesive beads, wherein the beads areelectrostatically chargeable, magnetically responsive, or a combinationthereof. An electrostatic charge may optionally be generated on thebeads. At least one of the following may be generated: an electrostaticcharge on a transfer medium opposite to the charge on the beads; anelectrical field, repulsive to the beads, encompassing the transfermedium wherein the electrical field is capable of positioning the beadsplaced within the electrical field on the transfer medium; a magneticfield encompassing the transfer medium. This preceeding step is optionalwhen the beads contain permanent magnetic particles and when thetransfer medium is magnetically responsive or when the beads containpermanent magnetic particles, the transfer medium is non-magneticallyresponsive, and a magnetically responsive material is positioned on aside of the non-magnetically responsive transfer medium opposite thebeads.

The beads are brought close to the transfer medium and/or the optionalcharge on the transfer medium and/or within the optional field(s)encompassing the transfer medium so that the beads contact the transfermedium and form a tack-free coating on the transfer medium. The beadsmay be activated so as to expose the pressure-sensitive adhesive coreand provide a coating of a pressure-sensitive adhesive on the transfermedium. The pressure-sensitive adhesive can then be transferred to asecond substrate to provide a coating of pressure-sensitive adhesive onthe second substrate.

Alternatively, one can bring a second substrate, which has beenoptionally charged with an electrostatic charge opposite to that on thebeads and/or which optionally has encompassing therearound a magneticfield into contact with the beads on the transfer medium, simultaneouslyactivating the beads so as to expose the pressure-sensitive adhesivecore and provide a coating of a pressure-sensitive adhesive on thesecond substrate.

Alternatively, one can transfer the beads from the transfer medium to asecond substrate and then subsequently activate the beads. One or moreof the following transfer methods can be used. One can bring a secondsubstrate which has been optionally charged with an electrostatic chargeopposite to that on the beads, wherein the optional charge on thesubstrate is optionally of greater magnitude than the charge that hasbeen optionally applied to the transfer medium close to the beads on thetransfer medium. One can bring a second substrate which optionally hasencompassing therearound a magnetic field of greater magnitude than thataround the transfer medium close to the bead(s) on the transfer medium.The beads are caused or allowed to contact the substrate and form atack-free coating on the substrate by one or more of the following: byan optional electrostatic charge on the second substrate; by an optionalmagnetic field encompassing the second substrate; by removing theoptional charge from the transfer medium; by removing the repulsiveelectrical field holding the beads to the transfer medium; by applyingan electrical force repulsive to the beads on a side of the transfermedium the transfer medium opposite the beads; by providing the secondsubstrate with an electrostatic charge greater than that has optionallybeen applied to the transfer medium. Subsequent to transfer to thesecond substrate the beads are activated to expose thepressure-sensitive adhesive core and provide a coating of apressure-sensitive adhesive on the substrate.

The PSA formed upon the activation of the beads of invention can becoated on a wide variety of substrates. Examples of substrates on whichthe beads of the invention or the adhesive prepared therefrom can becoated include but are not limited to those selected from the groupconsisting of paper, thermoplastic films, metal, cloth, wood,fiberglass, leather, glass, porous membranes, circuit boards.

The following is an example of a specific transfer process. A transfermedium is charged to a negative polarity via corona charging. Thetransfer medium is capable of retaining the charge on its surface for aworking period. Subsequently, positively charged pressure-sensitiveadhesive beads are brushed across the transfer medium surface and areattracted to the negatively charged areas. The greater the amount ofnegative charge in each area of the transfer medium the greater theamount of pressure-sensitive adhesive beads attracted thereto. Thesubstrate is brought close to the transfer medium. A negative chargegreater than that on the transfer medium is laid down on the surface ofthe substrate to attract the positive adhesive beads to the substratefrom the transfer medium. The substrate is then carried to a heated niproller which contacts the pressure-sensitive adhesive beads andactivates them on the substrate surface, thus forming a tacky adhesivesurface. Excess beads are vacuumed or blown clear.

The substrate may optionally be coated with a primer material prior tocoating with the unactivated beads of the invention or subsequent tocoating with the unactivated beads. The beads can be secured to thesubstrate simultaneously with positioning on the substrate or subsequentto positioning on the substrate. The beads, which are secured to thesubstrate, may then be activated at a later point in time prior to use.Primer materials are those defined as having the ability to maintain thePSA beads in position for a sufficient period of time to allow postactivation. Examples thereof include but are not limited to thoseselected from the group consisting of inks, shellacs, varnishes,adhesives, low melt temperature (co)polymers, polyolefins, and waxes,such as paraffin and beeswax.

Another method of application of PSA beads to a substrate involves theuse of a positioning particle(s). The positioning particle is a particlewhich is both magnetically responsive and electrostatically chargeable.Preferably, the positioning particle comprises a magnetically responsiveparticle having an electrostatically chargeable coating. Alternatively,the positioning particle comprises a magnetically responsive materialwhich is also electrostatically chargeable. The positioning particlescan be used as a template or positioning aid to positionpressure-sensitive adhesive beads on a substrate. The sameelectrostatically chargeable materials which can form the shell of thePSA beads can also be used to coat magnetically responsive particles toform one type of positioning particle. The same magnetically responsivematerials which can be included in the PSA beads can also be used informing the positioning particles. The diameter of the positioningparticle can vary according to use.

The method of application involves providing a first substrate and apositioning particle(s). The positioning particles are positioned on afirst substrate by a means selected from the group consisting ofelectrostatic force(s), magnetic force(s), both electrostatic andmagnetic forces to form a coating of the positioning particles on afirst substrate. A pressure-sensitive adhesive bead(s) is provided whichis electrostatically chargeable, magnetically responsive, or both. Theadhesive beads are then positioned on the positioning particle-coatedsubstrate by attraction and contacting the positioning particles andpressure-sensitive adhesive beads by a means selected from the groupconsisting of magnetic force(s), electrostatic force(s), both magneticforce(s) and electrostatic force(s) by bringing the bead(s) close to theparticles on the substrate. The beads can be activated on the substrate.Alternatively, the beads can be removed and positioned on a secondsubstrate by appropriate magnetic and/or electrostatic forces which donot remove the positioning particles themselves.

According to one specific positioning method, the pressure-sensitiveadhesive beads need not be magnetically responsive themselves as long asthe shell material is electrostatically chargeable. The particles can bepositioned on the substrate by means of a magnetic force(s). As oneexample, when the positioning particle comprises a permanent magneticparticle the magnetic attraction force can be provided by a magneticallyresponsive material in the substrate or on a side of the substrateopposite the positioning particles. The positioning particles arebrought close to the beads on the substrate thereby allowing themagnetic force(s) to position the positioning particles on the substrateto form a tack-free coating thereon. As another example, when thepositioning particle does not comprise a permanent magnetic particle amagnetic attraction force may be provided by a magnetic fieldencompassing the substrate. The positioning particles within themagnetic field are thereby positioned on the substrate to form atack-free coating thereon. Combinations of methods may also be employed.The magnetic force such as a magnetic field may be used to control thecoating pattern. The positioning particles can be placed close to or inthe magnetic field encompassing the substrate, wherein they contact thesubstrate in the desired pattern established due to the magnetic field.The electrostatically chargeable shell material on the PSA bead can beidentical to an electrostatically chargeable coating on the positioningparticle. Preferably, the positioning particle has an electron-donatingcoating and the pressure-sensitive adhesive bead has anelectron-accepting shell material. Alternatively, the positioningparticle preferably has an electron-accepting coating and thepressure-sensitive adhesive bead has an electron-donating shell. Thiscan be accomplished by selecting a different electrostaticallychargeable material for the PSA beads and for the positioning particles.A charge can be generated on the electrostatically chargeable PSA bead,on the positioning particles, or both. The ensuing electrostaticallyattractive force between the PSA beads and positioning particles resultsin the positioning of the pressure-sensitive adhesive beads on theparticle coated substrate.

According to this specific example, the electrostatically chargeableshell material of the pressure-sensitive adhesive bead(s) is attractedto and contacts the electrostatically chargeable coating of thepositioning particle which is being secured to the substrate by magneticforces. The electrostatically chargeable shell material of the adhesivebeads thus contacts the electrostatically chargeable coating of thepositioning particles resulting in a coating of the adhesive bead(s) onthe substrate. The beads can thereafter be activated on the substrate toform a coating of PSA on the substrate. Alternatively, the beads can beremoved by electrostatic forces. For example, a second substrate uponwhich an electrostatic charge of greater magnitude has been generatedwhich is attractive to the beads but not the positioning particles canbe brought close enough to the bead and positioning particle-coatedfirst substrate to attract the PSA beads and remove them viaelectrostatic forces. The beads can then be subsequently activated onthe second substrate.

Test Methods

Pourability of Beads--Funnel Discharge Time Test

Once dried, the pressure-sensitive adhesive beads according to theinvention are non-agglomerating, essentially cluster-free andfree-flowing. These terms are used interchangeably and are defined withreference to a modified American Society of Testing and Materials (ASTM)D-1895-69 with a funnel discharge time of less than 1 minute. Moreparticularly, a static free funnel having a volume of about 100milliliters and a discharge spout diameter of about 12.7 millimeters(ram) is employed. The discharge spout is temporarily closed by placinga static free object such as a wood tongue depressor against the distalend thereof and approximately 20 grams (g) of adhesive beads are lightlypoured into the funnel so as to avoid any packing thereof. The flatstrip of wood is removed and the elapsed time before the last beaddischarges from the funnel is recorded in seconds as the funneldischarge time.

This test method recognizes that small clusters or clumps of beads arepermissible within the scope of the invention so long as the clusters orclumps do not impede movement of the beads through the funnel to theextent that the funnel discharge time exceeds 1 minute. The shellcoating renders the beads non-tacky to the touch and contributes totheir free-flowing nature.

Peel Adhesion

The pressure-sensitive adhesive beads of Examples 2, 3, 5, and 11 wereeach separately tested for adhesion according to the followingprocedure. Standard, 20 pound (9.1 kg) bond white copier paper wascoated with an orange printing ink (Sinclair and Valentine #88318, St.Paul, Minn.) at a weight of 5.37 g/m² using a 2.5 inch (6.35 cm) rubberroller. The ink facilitated positioning and visualization of thesubsequently applied beads while maintaining the beads on the paperduring post treatment. While the ink was still slightly wet, it wasdusted with the previously dried, free-flowing adhesive beads. Excessadhesive beads were lightly shaken from the paper so as to provide amonolayer coating of beads thereon (approximately 139 g/m²). Theadhesive bead coated paper was air dried and cut into four equally sizedsamples. The adhesive coatings were then heat activated with a 4"×3" (20cm×7.6 cm), 2 kilogram heated plate assembly by exposing the coatedpaper samples to a temperature of 149° C. under a constant pressure of25.8 g/cm² for various times ("Dwell Time") as indicated in Table 2.

Peel adhesion of the pressure-sensitive adhesive beads to polyester filmwas determined according to a modified version of American Society ofTesting and Materials (ASTM) P3330-78, Method C. More particularly, astrip of double faced adhesive tape (Scotch™ brand No. 410 double coatedpaper tape commercially available from 3M, St. Paul, Minn.) was appliedto the steel panel of a 90 Degree Peel Jig (Chemsultants, Mentos, Ohio).The adhesive coated paper samples were applied to the double faced tapewith the adhesive bearing surface of the paper substrate facingoutwardly. A 1.25 inch (3.2 cm) by 0.9 mil (0.2 mm) polyester film wasapplied to the adhesive coated surface of the paper with two passes of aroller. One end of the polyester film was placed in the upper jaw of theadhesion tester and was pulled at a 90° angle relative to the steelplate and at the rate specified in the ASTM test method (30.5 cm/min).The peel adhesion value in grams per centimeter of width (g/cm width) isreported as the average of two samples.

Determination of Charge per Unit Area

The following is the test procedure used to determine the charge on thesurface of the free flowing pressure-sensitive adhesive beads of theinvention.

Procedure:

1) Prepare a test plate using a 102×102 mm by 4.7 mm thick glass platecoated with a thickness of less than 0.005 mm of an acrylate copolymeradhesive as described in U.S. Pat. No. Re 24,906, assigned to 3MCompany, St. Paul, Minn., incorporated by reference herein (a 95.5:4.5iso-octyl acrylate:acrylic acid copolymer of 1.5% solids in heptane),the purpose of which is to allow positioning of the adhesive beads to betested without their activation. The glass plate constructions weretested to determine the average dielectric constant at 100 Hz for use incalculations of charge density. This was accomplished followingspecifications outlined in ASTM D-150, titled A-C Loss Characteristicsand Permittivity (Dielectric Constant) of Solid Electrical InsulatingMaterials, incorporated by reference herein, using under sized lead foilelectrodes with brass dead weights, with a Hewlett Packard, (San Diego,Calif.) model #4284A, Induction, Capacitance, Resistance Meter, andmeasuring the dielectric constant at 100 Hz, 1 KHz, 10 KHz, 100 KHz, and1 MHz.

2) Mask off a 2 cm×2 cm square area in the center of the plate on theadhesive coated side using a TEFLON™ template (available from E. I.dupont de Nemours & Co.).

3) Grade the beads according to size using 425 and 500 micrometeropening sieves, and collect the beads retained in the 425 micrometerssieve. This gives a bead distribution of 425μ≦X≦500μ or 0.0165in.≦X≦0.0197 in. Charge 2 g of beads by placini the beads in a 100×15 mmpolystyrene petri dish with a non-conductive fiber board cover andshaking vigorously by hand for 30 seconds.

4) Quickly apply to the pre-masked area of the glass plate a monolayerof the charged beads. Apply a TEFLON™ release sheet (available from E.I. dupont de Nemours & Co.) and roll once using a 63.5 mm rubber rollerto tack the beads to the adhesive surface.

5) Using a Monroe Electronics, Inc., Lyndonville, N.Y., model #244miniature non-contact electrostatic voltmeter and a model #1015B probe,place the sample on the surface of a ground plate and adjust to a 1 mmgap between the surface of the beads and the probe end.

6) Slowly move the sample under the probe until the total surface areahas been sampled.

7) Record the voltage every 6 seconds.

8) Calculate the mean voltage over the 2×2 cm area.

9) Record the relative humidity and temperature during testing.

10) Calculate static charge density and static charge per bead basedupon average static volts and average bead diameter using the formulasbelow.

Static Charge Density=σ=.di-elect cons.V/4πrd

Static Charge/Bead=σ/η

Particle Density/Unit Area=η=1 cm² /4r²

.di-elect cons.=Dielectric Constant of the Adhesive Coated Glass Plate

d=Total Thickness of Plate in cm with Beads Coated Thereon

esV=Recorded Mean Static Volts

r=Average Radius of Beads in cm

1 Static Volt=300 Volts

1 Static Coulomb=3×10⁻⁹ Coulombs

Results are reported in Table 1 wherein the average PSA bead diameterwas 462.5μ.

Static Voltage v. Time

The following is the test procedure used to determine the static voltageversus time for the free-flowing pressure-sensitive adhesive beads ofthe invention.

(1) Charge 2 g of beads by placing the beads in a 100×15 mm polystyrenepetri dish with a nonconductive fiber board cover, shake vigorously byhand for 30 seconds.

(2) Quickly apply a monolayer of the beads to a standard ASTM 16 gaugestainless steel test plate.

(3) Using a Monroe, Electronics Inc., Lyndonville, N.Y., Model #244miniature noncontact electrostatic voltmeter and a Model #1015B probe,place the bead coated test plate on the surface of a ground plate andadjust to a 1 mm gap between the surface of beads and the probe end.

(4) Record the voltage every 15 seconds. Testing was conducted at 23.9°C. and 20% Relative Humidity.

EXAMPLES

The invention will be more fully appreciated with reference to thefollowing non-limiting examples. All parts, percentages, ratios, etc.,in the Examples and the rest of the Specification are by weight unlessindicated otherwise.

The following abbreviations and tradenames are used herein.

    ______________________________________                                        Abbreviation    Material                                                      ______________________________________                                        AA              Acrylic acid                                                  ACM             Acrylamide                                                    CBr.sub.4       Carbon tetrabromide                                           IOA             Isooctyl acrylate                                             IOTG            Isooctyl thioglycolate                                        IPA             Isopropyl alcohol                                             K.sub.2 S.sub.2 O.sub.8                                                                       Potassium persulfate                                          LiOH            Lithium hydroxide                                             MAA             Methacrylic acid                                              MMA             Methyl methacrylate                                           NaHSO.sub.3     Sodium bisulfite                                              NaLS            Sodium lauryl sulfate                                         NH.sub.4 OH     Ammonium hydroxide                                            NVP             N-vinyl-2-pyrrolidone                                         PMMA            Poly(methyl methacrylate)                                     PNVP            Poly(N-vinyl-2-pyrrolidone)                                   PS              Polystyrene                                                   S               Styrene                                                       SSS             Sodium styrene sulfonate                                      THF             Tetrahydrofuran                                               VAZO ™ 64*   2,2'-azobis(isobutyronitrile)                                 VOAc            Vinyl acetate                                                 ZnO             Zinc oxide                                                    % R.H.          percent relative humidity                                     Temp.           temperature                                                   Ex.             Example                                                       Sec.            seconds                                                       ______________________________________                                         *Commercially available under this tradename from E. I. duPont de Nemours     & Co., Wilmington, Delaware.                                             

Preparation of Pressure-Sensitive Adhesive Cores "A"

The following describes the preparation of pressure-sensitive adhesivecores "A" based on acrylic acid esters and using an aqueous suspensionpolymerization technique. The reaction was carried out in a five litersplit flask equipped with a condenser, a motor driven stainless steelstirrer having a speed control, a thermowell, a nitrogen gas inlet, andheating lamps with a temperature control. A dispersion of 7.8 g of ZnOand 1.56 g of PNVP in 1820 g of deionized water was added to the flask,the temperature was maintained at 58° C., and the agitator (stirrer) wasset at 375 revolutions per minute (rpm). A degassed monomer premixcomprising 5.2 g of MMA, 260 g of VOAc, 64.5 g of MAA, 1232.4 g of IOA,0.8324 g of IOTG, and 7.8 g of VAZO™64 2,2'-azobis(isobutyronitrile) wasthen added to the flask, followed by 3.9 g of NH₄ OH. An exotherm wasobserved during which time the temperature was maintained at about 68°C. with an ice-water bath. After 1.5 hours, the temperature was reducedto 65° C. and the agitation increased to 425 rpm. After 5.5 hours thetemperature was decreased to 50° C. and 3.12 g of LiOH in 40 milliliters(ml) of deionized water was added to the reaction flask. Agitation at425 rpm was continued for 0.5 hour. The resulting copolymerpressure-sensitive adhesive cores were dewatered and isolated at 70%solids by gravity filtration. Upon subsequent drying, thepressure-sensitive adhesive cores were inherently tacky and were notfree flowing as defined by the Funnel Discharge Time Test (ASTMD-1895-69) described above.

Example 1

Example 1 illustrates the formation of adhesive beads comprising apressure sensitive adhesive core surrounded by an essentiallydiscontinuous inorganic powder coating. More particularly, a dispersioncomprising 450 g of the filtered adhesive cores "A" and 450 g ofdeionized water was mixed with 12.6 g of AEROSIL® R972 hydrophobic fumedsilica (commercially available from Degussa Corp., Ridgefield Park,N.J.) dispersed in 27 g of IPA. The resulting mixture was heated at 65°C. with agitation for 30 minutes. The resulting beads were filtered anddried with constant agitation in a fume hood under ambient conditions.The resulting beads were free-flowing and had a moisture content of lessthan 1%. Charge per unit area was calculated and is set forth inTable 1. Peel adhesion and pourability was calculated and is set forthin Table 2. Static voltage versus time for the beads is shown in FIG. 1.

Example 2

Example 2 describes the formation of adhesive beads comprisingpressure-sensitive adhesive cores "A" surrounded by a continuous ureaformaldehyde shell. More particularly, a precondensate of the shellmaterial was prepared by mixing 48 g of urea and 12 1 g of 37% aqueousformalin and a sufficient amount of a 10% aqueous sodium hydroxidesolution to reduce the solution pH to 8.0. The mixture was agitated at70° C. for one hour. Upon formation of a linear formalin-urea polymer,28 g of the precondensate were added to a dispersion comprising 403 g ofthe filtered pressure-sensitive adhesive cores "A" and 500 g ofdeionized water. A sufficient amount of 5% aqueous hydrochloric acidsolution was added dropwise until the solution pH was reduced to 3.5.The resulting solution was agitated at 50° C. for about five hours. Thisprocedure was repeated until 103 g of the precondensate had beenconsumed in the formation of the adhesive beads. The coated beads werethen filtered and dried under ambient conditions. The resulting beadswere free-flowing. Charge per unit area was calculated and is set forthin Table 1. Peel adhesion and pourability was calculated and is setforth in Table 2. Static voltage versus time for the beads is shown inFIG. 1.

Example 3

Example 3 describes a polymeric material, for forming an essentiallydiscontinuous organic polymer coating which is provided as a granulatedpowder. More particularly, a reaction was carried out in a two litersplit flask equipped with a condenser, a motor driven stainless steelstirrer (agitator) having a speed control, a thermowell, heating lampswith a temperature control, and a nitrogen gas inlet. An aqueousdispersion comprising 10.0 g of PNVP, 297.0 g of MMA, and 3.0 g of SSSin 1000 g of degassed, deionized water was heated to 55° C. withagitation at 250 rpm. The flask was then charged with 0.60 g of K₂ S₂ O₈and the reaction was allowed to proceed for 4 hours at 55° C. Thereaction mixture was then cooled to room temperature (about 20°-22° C.)at which time a trace amount (about 0.01 g) of hydroquinone was added toremove any residual initiator. The reaction provided an organic polymerlatex to produce a polymeric material comprising 99 parts MMA and 1 partSSS. More specifically, the organic polymer latex was dried in an ovenmaintained at 65° C. for 15 hours and subsequently pulverized so as toform a dry, granular powder having an average particle size of less thanabout 1 micron in diameter. The polymeric material was subsequently usedto form an essentially discontinuous organic polymer shell about theadhesive cores "A".

12.6 g of the powder were combined with 450 g of the filteredpressure-sensitive adhesive cores "A" and 450 g of deionized water in atwo liter reaction flask. The mixture was heated to 65° C. withagitation and maintained at this temperature for approximately 30minutes. The adhesive beads were filtered and dried with constantagitation under ambient conditions. The resulting beads werefree-flowing. The beads had an essentially discontinuous organic polymercoating comprising 99 parts MMA and 1 part SSS. The beads had a moisturecontent of less than 1%. Static voltage versus time for the beads isshown in FIG. 1.

Example 4

Example 4 describes adhesive beads with a substantially continuousthermoplastic shell coating. More particularly, a reaction was carriedout in a two liter split flask equipped with a condenser, a motor drivenstainless steel stirrer (agitator) having a speed control, a thermowell,heating lamps with a temperature control, and a helium gas inlet. Asuspension was prepared from 343 g of adhesive cores "A", in 200 ml ofdeionized water containing 2 drops of AEROSOL® MA-80 (sodium dihexylsulfosuccinate surfactant) commercially available from American CyanamidCo., Wayne, N.J.) and 2 drops of POLYWET® Z-1766 (bisulfite terminatedsodium salt of polyacrylic acid commercially available from UniroyalChemical Co., Middlebury, Conn.). A redox initiator consisting of 0.1888g of potassium persulfate and 0.020 g of sodium bisulfite was added tothe suspension and the mixture heated to 70° C. under helium andconstant agitation at 350 rpm. MMA monomer was carefully added bysyringe pump according to the following schedule: 5 g at 5.1 ml/hr; 30 gat 8.4 ml/hr; and a final 10 g at 20 ml/hr by use of a dropping funnel.After 5 hr, the mixture was heated to 80° C. and another 15 g of MMAmonomer was added at 20 ml/hr also by dropping funnel. The adhesivebeads were filtered and dried under ambient conditions resulting in afree-flowing bead form with a moisture content of less than 1%.Photomicrographs of the beads showed a mainly continuous shell coatingwhich was free of discrete particles. Charge per unit area wascalculated and is set forth in Table 1. Peel adhesion and pourabilitywas calculated and is set forth in Table 2. Static voltage versus timefor the beads is shown in FIG. 1.

Example 5

Example 5 describes the preparation of magnetically responsive adhesivebeads. The reaction was carried out in a two liter split flask equippedwith a condenser, a motor driven stainless steel stirrer (agitator)having a speed control, a thermowell, heating lamps with a temperaturecontrol, and a nitrogen gas inlet. Prior to the reaction, PMMAhomopolymer was produced following the procedure of Example 4 with theexception of 300 g of MMA being used with no SSS. The PMMA emulsion wasthen dried in a 65.6° C. oven and ground by mortar and pestle toproduced a powder. The molecular weight as determined by gel permeationchromatography was an average molecular weight of 687,000 and apolydispersity of 3.0. The PMMA homopolymer powder was then dispersed inmethyl ethyl ketone at 10% by weight dry powder. To 50 g of 10% PMMAhomopolymer dispersion was added nickel powder (commercially availablefrom Inco Alloys International, Inc., Huntington, W. Va.) in the amountof 50 g, with a mean particle size of 50.45 microns (as determined byLeads and Northrup, Microtrac, Full Range Analyzer, North Whales, Pa.).The combined dispersion of PMMA and nickel was then dried in a 65.6° C.oven for 2 hours and ground by mortar and pestle. The powder was sievedto under 53 microns and then used in the following reaction. Adispersion of 1.5 g of ZnO and 0.30 g of PNVP in 350 g of deionizedwater was added to the reactor and the batch temperature was set to 58°C. with agitation at 375 rpm. A degassed monomer premix consisting of1.0 g of MMA, 50 g of VOAc, 12.3 g of MAA, 237 g of IOA, 0.1809 g ofIOTG and 1.5 g of VAZO™ 64 2,2',azobis(isobutyronitrile) was then added.After 1.5 hr., the batch temperature was reset to 65° C. and theagitation increased to 425 rpm. After 5.5 hr., the batch temperature wasreset to 50° C. 6.4 g of coated nickel powder and 6.4 g of micronizedpolyethylene were added. Agitation at 425 rpm was continued for 0.5 hr.The coated beads were then filtered off and dried with constantagitation in a fume hood under ambient conditions to give free-flowingbeads with a moisture content of less than 1%. Charge per unit area wascalculated and is set forth in Table 1. Static voltage versus time forthe beads is shown in FIG. 1.

Example 6

This example describes the preparation of adhesive beads using atackified rubber based adhesive system. More particularly, 128.9 g ofCA-501 rubber based adhesive (available from Century Adhesives Corp.,Columbus, Ohio) and 5.16 g of powdered homopolymer PMMA from Example 5were combined and frozen using liquid nitrogen. The frozen material wasthen ground by mortar and pestle to create particles of PMMA coatedadhesive. The resulting material was sieved to under 780 microns forfurther testing. Charge per unit area was calculated and is set forth inTable 1. Static voltage versus time for the beads is shown in FIG. 1.

                                      TABLE 1                                     __________________________________________________________________________    Determination of Charge per Unit Area                                         Example                                                                              Ex. 1 Ex. 2                                                                              Ex. 3                                                                              Ex. 4 Ex. 5                                                                              Ex. 6                                       __________________________________________________________________________    Avg. esV                                                                             -86.52                                                                              81.49                                                                              -71.17                                                                             -94.13                                                                              -46.91                                                                             276.33                                      Static Charge                                                                        -0.2357                                                                             0.2219                                                                             -0.1939                                                                            -0.2564                                                                             -0.1278                                                                            0.7527                                      per Bead                                                                      (esC/Bead)                                                                    Static Charge                                                                        -110.18                                                                             103.78                                                                             -90.63                                                                             -119.87                                                                             -59.74                                                                             351.90                                      Density                                                                       (esC/cm.sup.2)                                                                Temp.  22.8° C.                                                                     22.8° C.                                                                    22.8° C.                                                                    22.8° C.                                                                     22.8° C.                                                                    22.2° C.                             % R.H. 20%   20%  20%  20%   20%  20%                                         __________________________________________________________________________

Example 7

This example describes the preparation of adhesive beads according tothe invention wherein the coating is provided by a latex dispersion ofthe polymer additive that is added to the adhesive cores shortly afterthe observation of the exotherm which occurs during the aqueoussuspension polymerization of the cores.

More particularly, the reaction was carried out in a two liter splitflask equipped with a condenser, a motor driven stainless steel stirrer(agitator) having a speed control, a thermowell, heating lamps with atemperature control, and a nitrogen gas inlet. A dispersion of 1.5 g ofZnO and 0.3 g of PNVP in 350 g of deionized water was added to the flaskand the temperature was maintained at 58° C. with agitation at 375 rpm.A degassed monomer premix for forming the cores comprising 1.0 g of MMA,50 g of VOAc, 12.4 g of MAA, 237 g of IOA, 0.1606 g of IOTG, and 1.5 gof VAZO™64 2,2'-azobis(isobutyronitrile) was added, followed by 0.75 gof NH₄ OH. After the exotherm was observed (about 1 hour after the startof the reaction), 53.2 g of the organic polymer latex of Example 3 wasadded. After 1.5 hours, the temperature was increased to 65° C. withconstant agitation at 375 rpm. After 5.0 hours, the temperature wasdecreased to 50° C., and 0.16 g of LiOH in 10 ml of deionized water wasadded to the reaction mixture. Agitation at 375 rpm was continued for0.5 hour.

The adhesive beads were filtered and dried under constant agitation atambient conditions to give free-flowing beads with a moisture content ofless than 1%. The resulting beads were free-flowing. The beads compriseda pressure sensitive adhesive core surrounded by an essentiallydiscontinuous organic copolymer coating comprising 99 parts MMA and 1part SSS. Static voltage versus time for the beads is shown in FIG. 1.

Example 8

The procedure of Example 7 was repeated except that the organic polymerlatex of Example 3 was combined with the monomer premix for forming theadhesive cores prior to the initiation of the suspension polymerizationthat formed the cores. The resulting beads comprised a pressuresensitive adhesive core surrounded by an essentially discontinuousorganic polymer coating comprising 99 parts MMA and 1 part SSS. Theadhesive beads were filtered and dried under constant agitation atambient conditions to give free-flowing beads with a moisture content ofless than 1%. The resulting beads were free-flowing. This exampledemonstrates that adhesive beads according to the invention may beformed using an "in-line" process. Static voltage versus time for thebeads is shown in FIG. 1.

Example 9

An organic polymer latex was prepared using emulsion polymerizationaccording to Example 3 except that the two liter flask was charged with333 g of degassed and deionized water, 3.33 g of PNVP, and 100 g of MMA.Once the temperature reached 55° C., 0.202 g of K₂ S₂ O₈ and 0.145 g ofNaHSO₃ were charged to the reaction flask and the reaction was allowedto proceed for four hours at 55° C. Analysis by gel permeationchromatography indicated that the high molecular weight PMMA homopolymerlatex formed in this example had a weight average molecular weight of806,000 and a polydispersity of 3.3.

54.8 g of the high molecular weight PMMA homopolymer latex were added to450 g of the filtered adhesive cores "A" according to the procedure ofExample 3, thereby forming adhesive beads having an essentiallydiscontinuous organic polymer coating derived from a high molecularweight PMMA homopolymer. The adhesive beads were filtered and driedunder constant agitation at ambient conditions to give free-flowingbeads with a moisture content of less than 1%. The resulting beads werefree-flowing. Static voltage versus time for the beads is shown in FIG.1.

Example 10

An organic polymer latex was prepared by emulsion polymerizationaccording to Example 3 except that the two liter flask was charged with10 g of PNVP, 1000 g of degassed and deionized water, 300 g of MMA, and0.1620 g of CBh. Once the temperature reached 55° C., 0.60 g of K₂ S₂ O₈and 0.40 g of NaHSO₃ were added to the reaction flask and the reactionwas allowed to proceed for four hours at 55° C. After four hours at 55°C., 0.05 g of hydroquinone was added to deactivate any excess initiator.Analysis by gel permeation chromatography indicated that the lowmolecular weight PMMA homopolymer latex formed in this example had aweight average molecular weight of 687,000 and a polydispersity of 3.0.54.8 g of the low molecular weight PMMA homopolymer latex were added to450 g of the filtered adhesive cores of "A" according to the procedureof Example 3 thereby forming adhesive beads having an essentiallydiscontinuous organic polymer coating derived from a low molecularweight PMMA homopolymer. Peel adhesion and pourability was calculatedand is set forth in Table 2. The adhesive beads were filtered and driedunder constant agitation at ambient conditions to give free-flowingbeads with a moisture content of less than 1%. The resulting beads werefree-flowing. Static voltage versus time for the beads is shown in FIG.1.

Example 11

An organic polymer latex was prepared by emulsion polymerizationaccording to Example 3 except that the two liter flask was charged with10.0 g of NaLS, 0.132 g of CBr₄, 300 g of S, and 1,000 g of degassed anddeionized water. Once the temperature reached 55° C., 0.61 g of K₂ S₂ O₈was added and the reaction was allowed to proceed at this temperaturefor 4.5 hours. At this time, the reaction mixture was allowed to cool toroom temperature and 0.01 g of hydroquinone was added to remove anyresidual initiator. Analysis by gel permeation chromatography indicatedthat the low high molecular weight PS homopolymer latex formed in thisexample had a weight average molecular weight of 585,000 and apolydispersity of 2.4.

54.8 g of the low molecular weight PS homopolymer latex were added to450 g of the filtered adhesive cores "A" as described in conjunctionwith Example 3, thereby forming adhesive beads which included anessentially discontinuous organic polymer coating derived from a lowmolecular weight PS homopolymer. The adhesive beads were filtered anddried under constant agitation at ambient conditions to givefree-flowing beads with a moisture content of less than 1%. Theresulting beads were free-flowing. Static voltage versus time for thebeads is shown in FIG. 1.

                  TABLE 2                                                         ______________________________________                                        Dwell Time   Peel Adhesion (g/cm width)                                       (Minutes)    Ex. 1  Ex. 2      Ex. 4                                                                              Ex. 10                                    ______________________________________                                        0.5          1.0    8.4        0.7  83.8                                      1.0          2.2    11.7       0.7  114.6                                     3.0          4.1    26.8       2.8  130.8                                     10.0         35.2   41.3       4.3  256.9                                     Pourability  4.0    6.0        2.1  5.5                                       (sec.)                                                                        ______________________________________                                    

Example 12

To demonstrate the use of electrostatics in the coating of the pressuresensitive adhesives beads of the invention an experiment was conductedutilizing a Hipotronics Inc., Brewster, N.Y., High Voltage DC PowerSupply, model No. R10B, and the corona wire cartridge from a 3M Company,St. Paul, Minn., model No. 566A6, copy machine. The surface to be coatedwas a 15.24 cm wide by 1 mil thick polyester film, masked off by two 5.1cm strips of Scotch™ brand No. 811 removable Magic™ tape (commerciallyavailable from 3M Company, St. Paul, Minn.) leaving one 5.1 cm stripremaining running down the middle of the polyester film. 9000 volts dcwere applied to the corona cartridge and the polyester film with a 15.24cm wide paper carrier (20 pound, bond by Nekoosa, Ashdown, Ariz.) waspulled through the corona discharge at a rate of approximately 1.8 m perminute by hand, wherein the film and cartridge were separated by a 0.635cm air gap. A Monroe Electronics, Inc., Lyndonville, N.Y., Model No. 244miniature non-contact electrostatic voltmeter and a Model No. 1015Bprobe was used to determine the electrostatic voltage distribution overthe film surface. It was determined to be 2200-2400 electrostatic voltsin the non-masked areas and 530-860 electrostatic volts in the maskedareas, also determined that a similar distribution of negative polaritywas present on the opposite side. Several grams of the beads of each ofExamples 1, 2, 4, and 5 were separately charged by placing the beads ina 100×15 mm polystyrene petri dish with a polystyrene cover, and shakingvigorously by hand for 30 seconds. A monolayer of charged adhesive beadswere quickly applied to the film surface. Beads of Examples 2, 4 and 5had high bead concentrations in the area that was masked off duringcorona treatment, whereas the beads of Example 1 were concentrated inthe nonmasked area. For the beads of each of Examples 1, 2, 4, and 5some scattering of beads did occur outside their respective areas. TheMonroe non-contact voltmeter and a ground plate was used to determinethe polarity of the beads. The Monroe non-contact voltmeter and ModelNo. 1015B probe was placed on the surface of a ground plate and adjustedto a 1 mm gap between the surface of the beads and the probe end. Thesample was moved slowly under the probe until the total surface area hadbeen sampled. The pressure-sensitive adhesive beads of Examples 2, 4,and 5 were positive, whereas the pressure-sensitive adhesive beads ofExample 1 were negative. Thus, the coating patterns obtained from thebeads of Examples 2, 4, and 5 were reversed from that obtained from thebeads of Example 1.

Example 13

To demonstrate the use of magnetics in the coating of thepressure-sensitive adhesive beads of the invention, an experiment wasconducted utilizing the magnetically responsive beads prepared accordingto Example 5. A 21.8×21.6 cm piece of standard white medium bond paperwas placed on top of the adhesive-free side of 0.152 cm thick, 2.54 cmwide piece of 3M™ Brand No. 1317 magnetic tape (available from 3MCompany, St. Paul, Minn.) which had previously been adhered to a pieceof cardboard. The length of the magnetic tape was such that the edge ofthe magnetic tape extended beyond the edge of the standard white bondpaper. The beads of Example 5 were sprinkled onto the paper and magnetictape at their transition point. Excess beads were wiped off to provide amonolayer of beads. The beads oriented themselves in substantiallyparallel lines due to the magnetic field created by the underlyingmagnetic tape. FIG. 2 illustrates the parallel alignment of themagnetically responsive beads of the invention at the paper magnetictape interface, the paper constituting the light colored substrate andthe magnetic tape constituting the dark colored substrate. The beadsretained their pattern even when the composite was moved from ahorizontal position to a vertical position and even when the compositewas inverted.

Example 14

This example describes the use of non-incorporated magneticallyresponsive particles having an electrostatically chargeable coating(positioning particles) for the positioning of electrostaticallychargeable pressure-sensitive adhesive beads. Particles of PMMA coatednickel prepared according to the procedure of Example 5 were appliedaccording to the procedure of Example 13 to the standard white bondpaper with magnetic tape underlying. The PMMA/nickel positioningparticles oriented themselves in substantially parallel lines due to theunderlying magnetic tape. Low molecular weight polystyrene coatedadhesive beads prepared according to the procedure of Example 11 werecharged to a negative voltage by placing several grams of the beads in a100×15 mm polystyrene petri dish with a non-conductive fiber board coverand shaking vigorously by hand for 30 seconds.

The charged beads were then dusted onto the previously positionedPMMA/nickel positioning particles and the paper substrate. An inducedpositive charge was developed on the PMMA/nickel positioning particlescausing the PSA beads to be aligned between and around the PMMA/nickelpositioning particles. The resulting effect was a single stripe-coatingof the PSA beads.

This enables the aligned PSA beads to be activated in place to produce aPSA coating over the PMMA/nickel stripes, or in turn, to be transferredto a second substrate by utilizing a greater static electrical charge ona second substrate than that previously induced on the PMMA/nickelpositioning particles. The pressure-sensitive adhesive beads may then beseparated from the positioning particles and transferred to the secondsubstrate maintaining the pattern produced by the positioning particles.These may then be activated (by heat and/or pressure, for example) onthe second substrate.

Reasonable variations and modifications of the foregoing specificationare possible without departing from the scope of the invention which isdefined in the accompanying claims.

What is claimed:
 1. A coated substrate comprising a substrate having alayer of pressure-sensitive adhesive which displays pressure-sensitivetack coated thereon, wherein the pressure-sensitive adhesive is formedfrom free flowing adhesive beads wherein each adhesive bead consistsessentially of a pressure-sensitive adhesive core and a tack-free shelltherearound, wherein the pressure-sensitive adhesive core consistsessentially of a pressure-sensitive adhesive and wherein the beadconsists essentially of a magnetically responsive material and whereinthe beads are capable of being applied to the substrate via magneticforces, and wherein upon activation by heat and/or pressure the beadsform a layer of pressure-sensitive adhesive that displayspressure-sensitive tack on the substrate.
 2. The coated substrate ofclaim 1 wherein for each adhesive bead the adhesive core consistsessentially of a magnetically responsive material, the tack-free shellconsists essentially of a magnetically responsive material, or both theadhesive core and the tack-free shell consist essentially of amagnetically responsive material.
 3. The coated substrate of claim 1wherein each adhesive bead consists essentially of a pressure-sensitiveadhesive core and a tack-free shell therearound wherein the shellconsists essentially of a magnetically responsive material.
 4. Thecoated substrate of claim 1 wherein each adhesive bead consistsessentially of a pressure-sensitive adhesive core and a tack-freecontinuous shell consisting essentially of magnetically responsivematerial disposed about the core.
 5. The coated substrate of claim 1wherein for each adhesive bead the shell comprises a discontinuousshell.
 6. The coated substrate of claim 1 wherein for each adhesive beadthe shell comprises a continuous shell.